1. Field of the Invention
The present invention relates to an image projector to project images onto a screen or the like, and also relates to a light tunnel.
2. Description of the Related Art
Conventionally, an image projector is known such that, based on image data from a personal computer, a video camera and the like, images are formed by an image forming element using light emitted from a lamp, and the formed images are projected onto a screen or the like for display. In this kind of image projector, a light tunnel is used to make uniform the light emitted from the lamp, and the uniformed light is guided to the image forming element to form images.
A light tunnel has a light guide path in a form of a tube having an internal hollow. One end of the light guide path is a light entrance, while the other end is a light exit. The light emitted from the lamp enters the light guide path from the light entrance, and is uniformed and guided by being repeatedly internally reflected on the inner surfaces of the tubular light guide path. The uniformed and guided light is emitted from the light exit.
Generally, in such light tunnel, four glass plates are joined to form a light guide path, a light entrance and a light exit, with an outer shape being approximately rectangular parallelepiped. Each of the light entrance and the light exit has an end having- an outer shape of a rectangle, and has an aperture having a shape of a rectangle as well which is smaller in size, by the thickness of each glass plate, than the outer shape of the corresponding end thereof. The aperture of the light entrance has a size the same as that of the light exit. The light guide path has an aperture having a shape of a rectangle as well which is the same in size as the aperture of each of the light entrance and the light exit, and which has a constant size from the light entrance to the light exit.
On the other hand, in an optical device of a rod type in which light entering its light entrance end face is totally reflected, guided to its light exit end face and emitted from the light exit end face, it is known to provide a taper portion having a cross-section which continuously increases from the light entrance end face to the light exit end face, thereby reducing the divergence angle of light or beam emitted from the light exit end face. (Refer to e.g. Japanese Laid-open Patent Publication Hei 11-142780.) Further, in a light tunnel of a pipe type in which light entering into its hollow portion from its light entrance end is reflected, guided and emitted from its light exit end, it is known to make the cross-section of the light entrance end smaller than the cross-section of the light exit end, thereby making the exit angle of the light from the light exit end smaller than the entrance angle of the light to the light entrance end. (Refer to e.g. Japanese Laid-open Patent Publication Hei 10-48746. Furthermore, in an optical device of a rod type in which light entering its light entrance end face is totally reflected, guided to its light exit end face and emitted from the light exit end face, it is known to design the light entrance end face to have a form of a cone, thereby increasing uniformity of illuminance on the light exit end face. (Refer to e.g. Japanese Laid-open Patent Publication Hei 7-98479.)
However, the above-described conventional light tunnel has a problem as follows. As described above, the aperture of the light entrance is smaller in size, by the thickness of each glass plate, than the outer shape of the end of the light entrance. Accordingly, the light emitted from the lamp and irradiated onto the light entrance end is partially irradiated onto the end faces of the glass plates. The light irradiated onto the end faces of the glass plates does not enter the light guide path. In other words, the light irradiated onto the light entrance end face of the light tunnel is partially lost by not being able to entirely enter the light guide path, more specifically by being partially irradiated onto the end faces of the glass plates.
Accordingly, in an image projector using a conventional light tunnel, there has been a problem that it is not possible to display images by sufficiently utilizing light from a lamp, because the light from the lamp is partially lost by being partially irradiated onto end faces of glass plates forming the light tunnel. Besides, this problem cannot be solved even by applying the technologies disclosed in the above-referred references.